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Chapter 4
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Design Criteria
4.17 Street Drainage, Storm Drains and Sewers

4.17.1 Links to Standard Plans and Specifications and Other Resources
4.17.2 Stormwater Code Compliance
4.17.3 Stormwater Collection  

  4.17.4 Drainage and Sewer Conveyance
4.17.5 Side Sewers
4.17.6 Additional Information

Street design includes provision for the collection and discharge of storm water. Drainage system components such as pipe, catch basins, and inlets are considered integral street improvement elements as are curbs, sidewalks and pavement. All of the Street Drainage, Storm Drains and Sewer requirements in this section are to be considered requirements and are not optional.

When property development includes the installation of new or replaced paving or other impervious surfaces, there may be a need to improve existing drainage systems in the street to accommodate the additional stormwater or sanitary flows that will be incurred. This may trigger a requirement to make grading improvements, or to extend or upgrade existing storm drains, inlets, and catch basins, and there may be requirements for flow control and/or treatment facilities triggered by the City Stormwater Code. Factors such as the amount of impervious surface involved, the project location, the availability and capacity of existing infrastructure, among others, play a role in determining these requirements.

All these drainage improvements are required and shall be designed and constructed in accordance with the City of Seattle Standard Plans and Specifications which establish acceptable materials, dimensions, locations, installation and testing requirements, and other requirements for pipelines, manholes, connections, flow control facilities and other system improvements.

Seattle Public Utilities (SPU) is responsible for the plan review of new street drainage and stormwater treatment and flow control facilities, as well as for the review of project impacts to existing SPU infrastructure.

The following design criteria apply for storm drains and street drainage including drainage appurtenances, sanitary and combined sewers, and side sewers and service drains.
4.17.1 Links to Standard Plans and Specifications and other Resources

Standard Plans 200 Series
Standard Specifications Divisions 7
Standard Specifications Divisions 9
Stormwater Code SMC Ch. 22.800
Stormwater Flow Control and Water Quality Treatment Technical Requirements Manual DPD DR 17-2009/SPU DR 2009-005 Volume III
Green Stormwater Infrastructure – Requirements for Projects DR
Design Guidelines for Public Storm Drain Facilities SPU CAM 1180
Side Sewer Code – SMC Ch. 21.16
Requirements for Design and Construction of Service Drains (Drainage Water Discharges) DPD DR 3-2006/SPU DR 02-06
Requirements for Design and Construction of Side Sewers (Wastewater Discharges) DPD DR 2-2006/SPU DR 01-06
SPU Core Tap Procedures
Side Sewer Permits for Buildover Agreements DPD CAM 507

4.17.2 Stormwater Code Compliance

Project Type.  There are six types of projects identified with different minimum requirements for each.   See Section 2.3 of Volume 3 of the Stormwater Director’s Rules.  The 2009 Stormwater Code differentiates between projects on private property and projects in the ROW for the purposes of determining thresholds and minimum requirements.  All work in the ROW is treated as a separate project when determining minimum requirements.  As such all mitigation for work in the ROW must occur in the ROW and all mitigation for work on private property must occur on private property.  The two project types specific to the ROW are listed below:

  • Sidewalk Only – This involves only the creation, or replacement, of a sidewalk and only the associated work within the roadway prism needed to construct the sidewalk.
  • Roadway – This involves creation or replacement of an existing roadway.

Minimum Requirements for All Projects, Amended Soils:  All new, replaced and disturbed topsoil that will be left uncovered by impervious surfaces shall be amended with organic matter.  See Volume 3 Chapter 4 of the Stormwater Director’s Rules for detailed requirements.

Adding organic compost or mulch to soil improves its ability to support plants and absorb stormwater. Healthy soil is the backbone of green stormwater infrastructure.
Due to the concern that compost amended soils may compact irregularly, Seattle City Light (SCL) and Puget Sound Energy (PSE) require a 3-foot setback around all utility infrastructures.  In this 3-foot setback amending soils with compost is not allowed.  Seattle Public Utilities also recommends the 3-foot setback from water meters   Soils should also be compacted within one foot of a curb or sidewalk.

Drainage Review Thresholds: “Small Project” drainage review will be required for street improvements which involve more than 750 square feet of land disturbing activity but less than 5,000 sf of new plus replaced impervious surface and less than 7,000 sf of land disturbing activity.  “Large Project” drainage review will be required for street improvements which involve more than 5,000 sf of new plus replaced impervious surface or more than 7,000 sf of land disturbing activity.

Stormwater Flow Control : In areas served by a combined sewer, storm drains of inadequate capacity, or areas that discharge to non-designated receiving waters, stormwater flow control will be required for roadway projects that have a total new and replaced impervious areas exceeding 10,000 square feet, in accordance with the Stormwater Code . Stormwater flow control systems shall be either an off-line system or an in-line system as approved by SPU. Standard pipe material for flow control systems located within the City right-of-way and owned and operated by SPU shall be concrete or ductile iron pipe.

Stormwater Water Quality: If the new or replaced pollution generating impervious surface exceeds 5,000 square feet, except for areas that discharge to the combined sewer, there will be requirements for stormwater water quality.  (See City Stormwater Code for street specific impervious surface requirements). Public treatment facilities are installed in the right of way, and upon successful inspection they are turned over to SPU for operation and maintenance. Technical standards for the design of these facilities are provided in published SPU Director’s Rules.  

Green Stormwater Infrastructure:   All right-of-way projects with 7,000 square feet or more of land disturbing activity or 2,000 square feet or more of new plus replaced impervious surface must implement green stormwater infrastructure to infiltrate, disperse, and retain drainage water to the maximum extent feasible without causing flooding, landslide, or erosion impacts.  Technical standards for the design of these facilities are provided in published SPU Director’s Rules.
 
4.17.3 Stormwater Collection

Drainage: Shall be provided for improved streets and alleys and shall include catch basins and inlets that discharge to a public combined sewer, public storm drain, or other discharge point approved by Seattle Public Utilities (SPU).

Surface drainage: Establishing appropriate street grades is very important for drainage. Standards for cross slopes and longitudinal slopes are important for vehicular and pedestrian safety as well as surface water conveyance. A standard street cross section diagram can be viewed here and shall have a centerline crown elevation. Cross slope shall ensure surface drainage gets to the gutter and flows down to drainage pickups. Streets shall generally have a centerline crown elevation, with some exceptions, such as super-elevated streets.  Refer to Chapter 4.4 Grading and Chapter 4.5 Design Cross Section for more information.

  • Drainage shall be collected at the low point of all closed contours, the downstream end of developed alleys upstream of the sidewalk, and upstream of all ADA ramps. If the distance from a high point to the intersection, crosswalk, or end of an alley is less than 100 feet, drainage pickup may not be necessary.
  • Water from no more than 1,000 total lineal feet of curb may discharge into one catch basin.  This includes the length of curb for inlets which discharge into a catch basin as well as the catch basin itself.
  • At closed-contour low points and other locations where extra capacity is needed, use Standard Plan 242B catch basins. At low points in the roadway use two Standard Plan 242B catch basins, one on each side of the street, each with an independent connection to the storm drain. Additional drainage structure may be required where tree leaves are prevalent.
  • Existing inlets that do not conform to current standards must be replaced if located in areas where new full-depth pavement will be installed.  Existing inlets which do not conform to Standard Plan 250, located along new curbs, must be replaced.
  • Arterial streets with a grade flatter than 1% shall have drainage pickups (catch basins and/or inlets) at least every 350 feet. Gutter flow calculations may be required.
4.17.4 Drainage and Sewer Conveyance

Public storm drains: A new public storm drain may be required when

  • there is no available public storm drain or public combined sewer, and there is no acceptable discharge point;
  • to achieve adequate capacity; and

New public storm drains shall be sized to handle all upstream tributary area from the drainage basin in which they are located. They shall be designed to be continuous with existing and future storm drain pipes that are or will be part of the storm drainage system. The engineer shall provide the hydraulic calculations used in the pipe design for review by SPU. Refer to Chapter 6.4 Green Stormwater Infrastructure for guidance on providing a natural drainage conveyance system.

Storm drain pipes:

  • Storm drains must be designed for full gravity peak flow with 4% annual probability (25-year recurrence flows. The storm drain’s surcharge level (hydraulic grade line) for that peak flow may be no higher than four feet below the street’s gutter elevation or one foot below the service elevation of adjacent private property, whichever is lower. The service elevation is defined as two feet below the lowest elevation served on the site (such as the lowest catch basin or footing drain.)
  • If calculations show that a storm drain is surcharged, the Hydraulic Grade Line must be shown on the pipe profiles of the contract drawings. Use the high water elevation of the receiving waters to calculate hydraulic gradients.  High water for Lake Union and Lake Washington is +18.6; for Elliott Bay and the Duwamish River it is +12.14 (NAVD88).
  • Storm drains must be a minimum of 12 inches in diameter. The minimum slope for storm drains is 0.5% with 1.0% or greater being desirable.  Flatter slopes may be considered, but pipe slope must achieve a minimum velocity of three feet per second (fps) and shall require SPU approval.
  • The effects of excess energy shall be investigated whenever pipe velocity exceeds 20 feet per second (fps). Energy dissipation features may be required.  The design shall minimize or mitigate hydraulic jumps.
  • The crowns of all pipes shall match at the manholes. Invert elevations shall be calculated by projecting the pipe slopes to the center of the manholes.
  • Manholes are required every 375 feet.  Generally, manholes are required at pipe junctions, breaks in grade, and changes in horizontal alignment. When a small diameter storm drain intersects a very large diameter storm drain, it may be appropriate to set the manhole on the small diameter pipe 10 to 30 feet away from the junction. Manholes are needed at the end of all pipe runs unless the pipe is 100 feet in length or shorter.
  • Storm mains may have one horizontal or vertical bend (maximum 22.5°) between manholes, subject to approval by SPU.

Sanitary and Combined Sewers

In general, requirements for sanitary and combined sewer mains are as described above for storm drain pipes with the following differences:

  • All sewer pipe must be designed and constructed to give mean velocities, when flowing full, of not less than 3 fps. 
  • Pipe located in the ROW must have a minimum slope of at least 2%. Pipe slopes exceeding 50% requires the use of restrained joint ductile iron.  Pipes slopes that are <0.5% must have SPU approval.
  • Minimum pipe diameter is 8 inches
  • Manholes are required every 350 feet.
  • Minimum pipe depth is 12 feet.
4.17.5 Side Sewers

Side sewers and service drains:  The pipeline between the building and the sewer or storm drain main is a side sewer or service drain, respectively.  All side sewer and service drain work within the right-of-way requires a Street Use Utility Permit issued by DPD on behalf of SDOT.

Minimum grade, pipe size and materials, connection details, installation and testing requirements:  Side sewers and service drains shall be designed and installed in accordance with the City of Seattle Standard Plans and Specification. New core taps shall be per SPU’s Core Tap Procedure

Ownership:  Side sewers and service drains are owned and maintained by the property owner. The minimum grade is 2%.  Pipe material and size shall be according to the Seattle Side Sewer Code.  

Use of existing side sewers:  It is possible to use existing side sewers in lieu of a new connection in some cases See the Side Sewer code for more details.
4.17.6 Additional Information

Build-Overs: For build-overs for SPU owned sewer and drainage appurtenances, the applicant shall follow the guidance of DPD’s build-over CAM.

Ditch Modifications:

The City does not permit the filling of a ditch if that ditch functions as part of the City’s informal drainage system in the street right of way and is located within a creek watershed.  Creek watersheds are identified on the City GIS system, and on the Seattle Creek Watersheds map.  The SDOT Director may approve a requested exception per the Street Use Code SMC 15.04.112 if the Director determines that the modification is likely to be equally protective of public health, safety and welfare, the environment, or public and private property. If the proposed modification is not equally protective, the Director may approve a requested exception if substantial reasons are documented such as:

  • An emergency situation necessitates approval of the exception;
  • A reasonable use of the adjacent property is not possible unless the exception is approved; or harm or threat of harm to public health, safety and welfare, the environment, or public and private property is at risk unless the exception is approved.

Exceptions to the ditch filling moratorium policy include culvert installations that are necessary to implement driveway permits, and required street improvements. If you are considering modifying the ditches within a creek watershed, the City encourages use of an NDS approach. The capacity calculations will be required to show that the proposed system can, at a minimum, provide the level of service of the existing conditions.

Deviation from Drainage and Wastewater Standards: All requests for exceptions to the drainage and wastewater standards and requirements must be approved by the Drainage and Wastewater Appeals Board.
continue to section 4.18 »   
Latest Online Manual
Detailed Table of Contents
Chapter 4
Design Criteria
4.1 Introduction
4.2 Street Classifications and Street Types
4.3 Design Criteria General Notes
4.4 Grading
4.5 Design Cross Section
4.6 Roadway Width
4.7

Roadway Pavement

4.8 Intersections
4.9 Driveways
4.10 Curbs
4.11 Sidewalks
4.12 Crosswalks
4.13 Bicycle Facilities
4.14 Street Trees and Landscape Architectural Standards
4.15 Introduction to Utilities Design Criteria
4.16 Street Lighting
4.17 Street Drainage, Storm Drains and Sewers
4.18 Water Mains
4.19 Fire Protection
4.20 Seattle City Light
4.21 Clearances
4.22 Structures in the Right-of-Way
4.23 Culdesacs and Turnarounds
4.24 Traffic Operations
4.25 Transit Zones
4.26 Street Furniture, Public Art and Unique Objects in the Public Right-of-Way
4.27 Access Easements
4.28 Contact Information
   
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